Since intl-relativetimeformat 6.0, the locales are no longer located in the dist subfolder (https://github.com/formatjs/formatjs/blob/main/packages/intl-relativetimeformat/CHANGELOG.md#600-2020-07-03). As a consequence, this line has no effect:

https://github.com/wiziple/gatsby-plugin-intl/blob/master/src/gatsby-node.js#L29

new webpack.ContextReplacementPlugin(
        /@formatjs[/\\]intl-relativetimeformat[/\\]dist[/\\]locale-data$/,
        regex
      )

The fix is probably to just remove the dist[/\\] part but I haven't tried.